Year-round movements of sympatric Fork-tailed (Oceanodroma furcata) and Leach's (O. leucorhoa) storm-petrels

Long-distance movements are characteristic of most seabirds in the order Procellariiformes. However, little is known about the migration and foraging ranges of many of the smaller species in this order, especially storm-petrels (Hydrobatidae). We used Global Location Sensors to document the year-round movements of sympatrically breeding Fork-tailed Storm-Petrels (Oceanodroma Arcata) and Leach's Storm-Petrels (O. leucorhoa) from the Gillam Islands located northwest of Vancouver Island, British Columbia, Canada. In 2016, breeding Fork-tailed (N = 5) and Leach's (N = 2) storm-petrels traveled maximum distances of similar to 1550-1600 km from their colony to a region that has a wide shelf with major canyons creating a highly productive foraging area. After the breeding season, Fork-tailed Storm-Petrels (N = 2) traveled to similar areas west of the Gillam Islands, a maximum distance of similar to 3600 km from the breeding colony, and remained in the North Pacific Ocean and north of the Subarctic Boundary for an average of 5.4 mo. Post-breeding Leach's Storm-Petrels (N = 2) moved south to the Eastern Tropical Pacific, west of central Mexico, Ecuador, and northern Peru, an estimated maximum distance of similar to 6700 km from their breeding colony, and remained there for an average of 7.2 mo. Carbon (delta C-13) and nitrogen (delta N-15) stable isotope analyses of feathers revealed niche separation between Fork-tailed (N = 21) and Leach's (N = 53) storm-petrels. The wide range of delta N-15 values in the feathers of Leach's Storm-Petrels (N = 53) suggests that they foraged at a variety of trophic levels during the non-breeding season. Our results demonstrate that storm-petrels have large core foraging areas and occupy vast oceanic areas in the Pacific during their annual cycle. However, given the coarse precision of Global Location Sensors, additional study is needed to identify the specific areas used by each species during both breeding and non-breeding periods.